In high-speed data communications systems, one of the major limitations to the rate at which data can be reliably transmitted through a channel is the band-limited nature of the channel. The band-limiting of the channel produces a low pass filtering effect that results in symbols being dispersed in time at the receiver. The time dispersal can cause symbols to overlap. For example, the trailing edges (or “tails”) of preceding symbols can overlap the designated time slot of a succeeding symbol. The overlapping effect is termed Inter-Symbol Interference (ISI) and can result in significant “eye-opening” reduction (where eye-opening is a measure of data reception reliability) at the receiver.
An approach to mitigating ISI is to include a linear equalizer in the receiver. Linear equalizers can operate as follows. The high-frequency part of an input signal is separated out, amplified and then added back to the input signal. In this way, the high-frequency portion of the input signal receives greater gain relative to the lower frequencies.
A limitation of the linear equalizer approach, however, are the high-gain, wide-band, amplifiers used to gain-up the high-frequency portion of the input signal. It is difficult to realize accurate equalization with such amplifiers because of such factors as: phase mismatch between the input and high-frequency, gained-up, signal paths; and difficulty controlling the RC time constant at the input of the high-frequency amplifiers. Additionally, high-frequency amplifiers can consume significant power and accentuate high-frequency noise sources (such as crosstalk).
Thus, while it is desirable to have receiver equalization, it is difficult to do so in an accurate and power-efficient manner.